You'd have to be quite fat to fit the airline's requirements for a double ticket. I almost never see people that fat on flights, but I don't fly very much.
That being said this will never fly.They won't have the balls to enforce it, and if they do, they are going to lose business.
How stupid are they to make a press release that says they are going to discriminate against a certain selection of their clientele? What a great way for a financially insolvent airline to go from hated to despised.